1. Field
The present description pertains to managing effects of discarded service data units (SDUs) during handover, and maintaining order of arrival of Packet Data Convergence Protocol (PDCP) SDUs when a UE encounters a lost PDCP.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and the like. Such systems can be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
The General Packet Radio Services (GPRS) system is a ubiquitous mobile phone system is used by GSM Mobile phones for transmitting IP packets. The GPRS Core Network (an integrated part of the GSM core network) is a part of the GPRS system that provides support for WCDMA based 3G networks. The GPRS Core Network can provide mobility management, session management and transport for Internet Protocol packet services in GSM and WCDMA networks.
GPRS Tunneling Protocol (GTP) is an IP protocol of the GPRS core network. GTP can enable end users of a GSM or WCDMA network to move from place to place while continuing to connect to the Internet as if from one location at a particular Gateway GPRS Support Node (GGSN). Such is accomplished by carrying subscriber's data from a subscriber's current Serving GPRS Support Node (SGSN) to the GGSN that is handling the subscriber's session. Three forms of GTP are used by the GPRS core network including (1) GTP-U: for transfer of user data in separated tunnels for each PDP context; (2) GTP-C: for control reasons such as setup and deletion of PDP contexts and verification of GSN reachability updates as subscribers move from one SGSN to another; and (3) GTP′ for transfer of charging data from GSNs to the charging function.
GPRS Support Nodes (GSN) are network nodes that support the use of GPRS in the GSM core network. There are two key variants of the GSN including Gateway GPRS Support Node (GGSN) and Serving GPRS Support Node (SGSN).
A GGSN can provide an interface between the GPRS backbone network and the external packet data networks (radio network and the IP network). It can convert GPRS packets coming from the SGSN into the appropriate packet data protocol (PDP) format (e.g. IP or X.25) and send the converted packets them to the corresponding packet data network. In the other direction, PDP addresses of incoming data packets may be converted to the GSM address of a destination user. The readdressed packets can then be sent to the responsible SGSN. For this purpose, the GGSN can store the current SGSN address of the user and his or her profile in its location register. The GGSN can provide IP address assignment and is generally the default router for a particular UE.
In contrast, an SGSN can be responsible for the delivery of data packets from/to mobile stations within its geographical service area. The tasks of an SGSN can include packet routing and transfer, mobility management, logical link management, authentication and charging functions.
Moreover, the GPRS tunneling protocol for the user plane (GTP-U) layer may be used on the user-plane (U-plane) and is useful for transmitting user data in a packet switched area. Packet switched networks in the Universal Mobile Telecommunications System (UMTS) are based on GPRS, and therefore, the GTP-U may also be used in the UMTS. UMTS is one of the third-generation (3G) cell phone technologies. UMTS is sometimes referred to as 3GSM, which hints at both its 3G background and the GSM standard for which it was designed to succeed.
3GPP Long-term evolution (LTE) complements the success of High Speed Packet Access (HSPA) with higher peak data rates, lower latency and an enhanced broadband experience in high-demand areas. This is accomplished with the use of wider-spectrum bandwidths, Orthogonal Frequency-Division Multiple Access (OFDMA) and SC-FDMA (i.e., single carrier) air interfaces, and advanced antenna techniques. Such techniques enable high spectral efficiency and an excellent user experience for a wide range of converged IP services. UMTS operators are rapidly adopting and offering IP services such as rich multimedia (e.g., video-on-demand, music download, video sharing), VoIP, PTT and broadband access to laptops and PDAs. Operators offer these services through access networks such as HSPA, HSPA+ and LTE.
It should be appreciated that there will be instances where a one Node-B (or more appropriately for these particular telecom standards “eNB”) will hand communication off to a second eNB. For the purpose of this disclosure, the eNB losing communication with a UE may be referred to as the “source eNB” while the eNB gaining access to the UE may be referred to as the “target eNB.”
For Long Term Evolution (LTE) communication systems, it can be beneficial to guarantee that downlink Radio Link Control (RLC) service data units (SDUs) are delivered “in-order” during handover. LTE communications systems, such as UMTS) can use PDCP as one of the layers of the Radio Traffic Stack. PDCP can perform a variety of functions including IP header compression and decompression, transfer of user data and maintenance of sequence numbers (SNs).
During handover, a target eNB may receive packets from two sources including an X2 source (e.g., from another eNB, such as the source eNB) and an S1 source (e.g., from a node of the supporting communications backbone). During handover, the target eNB can assign the PDCP sequence number (PDCP-SN) to those packets correctly to ensure they are delivered in-order at the UE—advantageously with only a minimum DL data delay.
In LTE, the relevant specifications promote that PDCP SDUs be delivered in-order to the upper layer above PDCP if so configured. As such, PDCP SDUs should typically be delivered to the layer above PDCP in the same order they arrive at the Serving Gateway (for DL) or the UE (for UL. The current baseline solution in RAN3 (i.e., TSG RAN working group 3) for handover is that during handover, the source eNB can provide the “next PDCP SN to use” to the target eNB. After that, the source eNB may freeze the PDCP SN, not assign any new PDCP SN to the SDUs, and forward all those SDUs to the target eNB without a PDCP SN. For all the SDUs that have a PDCP SN, the source eNB may try to send them to the UE and if not successful, forward those to the target eNB (unciphered) with the PDCP SN attached.
In various system arrangements, the target eNB may receive two SDU streams—one from the source eNB (via the X2 interface) and the other one from the SGW (via a new S1 interface). To ensure in-order delivery, the target may need to assign the “next PDCP SN to use” (N) to the first forwarded PDCP SDU without a SN. Since the target eNB does not know if such SDUs even exist, the target eNB may have to wait for up to some predetermined time limit. If no SDUs arrive from the source eNB after the predetermined time limit has past, the target eNB may assign N to the first PDCP SDU coming from the SGW. If there are SDUs coming from the source eNB, the target eNB may serve those first before serving the SDUs from the new S1. At some point, if there are no more SDUs coming from the source eNB, the target will start to serve the SDUs from the new S1. Any forwarded SDUs after that may be discarded or transmitted to the UE as well.
The problems with the RAN3 proposed baseline solution are numerous. For example, if the predetermined time limit is set too long, there is unnecessary waiting (fixed delay all the time). On the other hand, if the predetermined time limit is set too short, any SDUs arriving after the predetermined time limit expires will be either transmitted to the UE intermixed with the SDUs from the new S1 (SDU out-of-order delivery); or discarded (bad for TCP/IP). Further, since the optimum timer value will depend on a lot of factors (e.g., backhaul load), it may be hard to use a fixed value.
In addition, since the target can be holding the SDUs coming from the new S1 (until it is sure no more SDUs are being forwarded from the source eNB), the target eNB may be under utilizing the over-the-air bandwidth when related the predetermined time limit timer is active. Moreover, the SDUs can receive data out-of-order within S1 and X2, regardless of handover.